Concrete jacket construction detail effectiveness when strengthening RC columns

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Construction and Building

MATERIALS

Construction and Building Materials 22 (2008) 264–276

www.elsevier.com/locate/conbuildmat

Concrete jacket construction detail effectiveness when strengthening RC columns Konstantinos G. Vandoros, Stephanos E. Dritsos

*

University of Patras, Department of Civil Engineering, 26500, Patras, Greece Received 14 February 2006; received in revised form 5 August 2006; accepted 30 August 2006 Available online 23 October 2006

Abstract This paper presents an experimental investigation of the effectiveness of strengthening half height full size concrete columns by placing concrete jackets. Three alternative methods of concrete jacketing are investigated and results are compared with results from an original unstrengthened specimen and a monolithic specimen. The specimens were designed to represent typical ground floor columns of a concrete frame building. The unstrengthened column and the original columns of the strengthened specimens were designed to old 1950s Greek Codes. Poured concrete or shotcrete was used to construct the jackets of the strengthened specimens and, as performed in practice, various other construction procedures were carried out in order to evaluate if it is worth performing the procedures when considering the practical difficulties involved. These procedures involved welding the jacket stirrup ends together, placing steel dowels across the interface between the original column and the jacket in combination with welding the jacket stirrup ends together and connecting the longitudinal reinforcement bars of the original column to the longitudinal reinforcement bars of the jacket. In order to investigate the lower limit of the effectiveness of the technique, the case of no treatment at the interface between the original column and the jacket combined with the construction of a low strength cast in situ concrete jacket is examined. The same cross sectional dimensions and amount of steel reinforcement were used for the strengthened specimens and a control monolithic specimen. Earthquake simulation displacement controlled cyclic loading was used for the testing. The seismic performance of the tested specimens is compared in terms of strength, stiffness and hysteretic response. The effectiveness of properly constructing concrete jackets has been proved, as it was found that, under special conditions, an almost monolithic behaviour could be achieved. Even when the jacket was constructed with no treatment at the interface, a significant strength and stiffness increase was observed. It was also found that the failure mechanism and the observed crack patterns are influenced by the strengthening method. The separation of the jacket from the original column was obvious in the case when there was no treatment or other connection means performed at the contact interface between the column and the jacket. In addition, it was found that welding the jacket stirrup ends together stopped the longitudinal bars of the jacket from buckling.  2006 Elsevier Ltd. All rights reserved. Keywords: Concrete columns; Shotcrete; Strengthening; Retrofitting; Jacket; Stirrup end welding; Dowels; Steel connectors and seismic performance

1. Introduction It is known that many buildings designed to older codes may be susceptible to serious damage during a large earthquake. Older buildings have been structurally designed for much lower seismic actions when compared to buildings that are designed today. This is because the relevant seismic *

Corresponding author. Tel.: +30 2610 997780; fax: +30 2610 997575. E-mail address: [email protected] (S.E. Dritsos).

0950-0618/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2006.08.019

codes have been continually revised as knowledge has increased. One popular solution to the problem of how to strengthen old reinforced concrete (RC) structures is to place jackets around the structural elements. Jackets have been constructed using concrete, steel elements and fibre-reinforced polymer composites. A variety of techniques have been used to strengthening beams, columns and joints by placing concrete jackets. Several experimental studies [2,4–6,9–11,13,14] have been performed in order to investigate the method. It has been shown that the method

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276

construction of a low strength poured concrete jacket was investigated. 2. Experimental work Three different strengthening techniques were used to construct half height full-scale ground floor columns. The three techniques were: welding the jacket stirrup ends together (denoted as N), placing dowels and jacket stirrup end welding (denoted as E) and placing bent down steel connector bars welded to the original column longitudinal reinforcement bars and the respective bars of the jacket (denoted as W). For all strengthened specimens, the surface of the original column was not roughening in order to eliminate the influence of roughening when assessing the effectiveness of the methods. Poured concrete was used for specimens N and E and shotcrete was used for specimen W. Initially for each specimen, as shown in Fig. 2, an original column was constructed on a strong foundation. The concrete for each original column and foundation was placed at the same time. The original columns were designed to simulate usual detailing deficiencies. These deficiencies were the use of mild steel reinforcement, widely spaced stirrups and inadequate hooks at the ends of the stirrups. The foundation of every specimen was heavily reinforced. The reinforcement consisted of 16 mm diameter grade S500 bars spaced at every 150 mm in three directions. The dimensions of the foundations were 1400 mm by 780 mm by 650 mm (Fig. 2). The procedure and detailing used to manufacture each original column was identical to that of an unstrengthened specimen (denoted as O) that was constructed for a previous study at the Structural Laboratory of the Department of Civil Engineering at the University of Patras [3]. As shown in Fig. 3, the original columns had cross sectional dimensions of 250 mm by 250 mm. Four 14 mm diameter grade S220 bars were used for the longitudinal reinforce-

1800 mm

improves the bending strength, the shear capacity, the stiffness, the ductility and the axial load carrying capacity of strengthened elements. In practice, a variety of procedures are used to construct concrete jackets around RC columns [7]. Although the method has a widespread use, an experimental investigation that compares the different ways of executing the techniques has not yet been reported. The question to be answered was how the effectiveness of the technique alters when common as used in practice procedures are performed. These procedures include: (a) using bent down bars to connect the jacket bars to the longitudinal reinforcement of the original column, (b) placing dowels at the interface between the existing column and the jacket, (c) welding together the jacket stirrup ends in order to offset the unavoidable inability to fulfil construction detailing requirements of existing codes and (d) the method of application of the jacket material which may be sprayed shotcrete concrete or poured concrete. Procedure (a) is a traditional Greek practice and is shown in Fig. 1. This procedure is difficult to perform and it has been investigated in order to examine if it is worthwhile practice. The other aim of the present work was to investigate the lower limit of the effectiveness of constructing concrete jackets. In practice, it is not always possible to guarantee the quality control and there is a need to investigate what happens when jackets are constructed under the worst conditions. For this reason, the procedure of no treatment at the interface between the original column and the jacket in combination with the

265

N

650 mm

575

E

265

W

780 mm 250 265

575

1400 mm 250

S Plan Fig. 1. Bent down bars as used in practice.

1400 mm

Facade N and S

Fig. 2. Original column.

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276 200 mm

266

Φ10/100 (S500) Φ 20 (S500)

Φ14 (S220)

1300 mm 75 mm

Original column

1600 mm

75 mm

10 55 20 55 10

230

Jacket

250 mm

m

20

100 m

Φ 8/200 (S220)

a) Cross-section

b) Facade S and N

Fig. 3. Geometrical characteristics of the strengthened elements.

ment and each bar was anchored in the foundation by using 180 hooks. Stirrups of 8 mm diameter grade S220 bars were spaced at every 200 mm and the stirrup ends were 90 hooks. The concrete cover was 10 mm. In order to apply the horizontal load, the tops of the columns were locally strengthened. Four 18 mm diameter grade S500 bars and 8 mm diameter grade S220 stirrups at 100 mm spacing were used for the local strengthening. In addition, a 10 mm diameter grade S500 U-shaped bar was placed in each side. In order to attach the horizontal displacement actuator, 40 mm diameter plastic pipes were placed near the top of each column. For all strengthened specimens, the thickness of the jacket of was 75 mm and there was no special treatment at the interface between the original column and the jacket. The reinforcement for each jacket consisted of four longitudinal 20 mm diameter grade S500 bars and 10 mm diameter grade S500 stirrups spaced at every 100 mm. In general, it was not possible to form 135 hooks at the ends of the stirrups, since the original column impeded the hooks and the jacket stirrup ends were bent in towards the concrete core as far as possible. In some cases, as described below in more detail, the stirrup ends were welded together to ensure their anchorage. The concrete

cover of the jacket was 20 mm. The longitudinal bars of the jacket were placed from the beginning. They were anchored in the foundation by using 200 mm long 90 hooks. Each jacket was constructed to a height of 1300 mm above the foundation. After casting the jacket, the final dimensions of the cross section were 400 mm by 400 mm. One month after casting the concrete of the original column, the jacket stirrups were placed and the jacket was constructed by using either shotcrete or cast in situ concrete. The monolithic specimen had the same cross sectional dimensions and the same longitudinal and stirrup reinforcement as the strengthened specimens (Fig. 3). The mechanical characteristics of the s teel used for all specimens are presented in Table 1. Table 2 presents the mean value of the cylindrical concrete strength on the day of testing for all specimens. The construction detailing of the jackets was different for each strengthened specimen. For specimen N, the ends of the four lowest stirrups of the jacket were welded together. The weld length was 50 mm, as shown in Fig. 4. For specimen E, holes of 22 mm diameter were drilled in every side of the original column at heights of 200 mm, 700 mm and 1200 mm above the foundation. A special

Table 1 Steel characteristics Element

Steel grade

Bar diameter

Yield stress (MPa)

Ultimate stress (MPa)

Original column

Longitudinal reinforcement Stirrups

S220 S220

U14 U8

313.0 425.4

441.7 596.3

Jacket

Longitudinal reinforcement Stirrups

S500 S500

U20 U10

487.1 599.2

657.0 677.2

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276 Table 2 Concrete strengths

267

Table 3 Strengthened specimens characteristics N

E

W

O

M

Specimen

N

E

W

Original column concrete strength (MPa) Jacket concrete strength (MPa)

27.0 17.8

36.8 24.0

22.9 18.8

27.0 –

24.7 –

Jacket concrete Dowels Bent down bars Stirrup ends welding

Poured No No Yes

Poured Yes No Yes

Shotcrete No Yes No

360 mm

50 mm

Specimen

360 mm Fig. 4. Welded jacket stirrup geometry.

resin was injected into the holes before placing 20 mm diameter grade S500 L-shaped dowels of dimensions 150 mm by 100 mm. The long branch of the dowel was placed 100 mm into the holes and, after placing, protruded 50 mm from the original column. The dowels were placed after the jacket stirrups had been placed. In addition, the ends of the four lowest stirrups of the jacket were welded together, as was performed for specimen N. For specimen W, special bent down steel connectors were welded between the longitudinal reinforcement bars of the original column and the jacket. The steel connectors were 16 mm diameter (grade S500) reinforcement bars, as shown in Fig. 5. The steel connectors were placed in every

Original column bar

corner of the specimen at heights of 250 mm, 700 mm and 1100 mm above the foundation. In total, 12 steel connectors were placed. The placement procedure of the steel connectors was as follows: the corner concrete cover of the original column was chipped off until the original column bar was revealed, the steel connector was first welded to the original column bar (at one point) and then to the jacket bar (at two points). As also shown in Fig. 5, each weld length was 70 mm, which was 5 times the diameter of the thinnest bar to be welded together. Welding was performed only on one side of the bars. Table 3 presents a summary of the characteristics of the strengthened specimens. 3. Test procedure The same test procedure was used for all specimens. Each specimen was first moved to the testing area and anchored to a strong floor. A hydraulic jack and an IPE 600 steel beam were used to apply a constant axial load to the top of each specimen. Each test was initiated by applying a displacement controlled horizontal cyclic load to the top of the unjacketed part of the column. The lateral displacement of the column was also measured at this point. A testing sequence, similar to one used in other tests at the Structural Laboratory of the University of Patras, consisted of imposing a displacement of 5 mm for the first cycle and then increasing the displacement by 5 mm for all further cycles. The applied displacement for each test is shown in Fig. 6 and was applied in the E-W direction (Fig. 2 above). The test set up is presented in Fig. 7.

Jacket bar 150

13

5o

100

welding 70 mm

welding 70 mm

Displacement (mm)

150 mm

100 mm

150 mm

welding 70 mm

50

0

-50

-100

-150 0

5

10

15

Cycle number Fig. 5. Bent down steel connector geometry.

Fig. 6. Lateral displacement history.

20

25

268

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276

Fig. 7. Test set up.

The normalized axial load ratio for the strengthened specimens was calculated from the following expression: mi ¼

Ni ðAco  fco Þ þ ðAcj  fcj Þ

The normalized axial load ratio for specimens O and M was calculated using the following equation: mi ¼

Ni Aco  fco

where: Ni Aco

Acj fco fcj

is the initial applied axial load, is the cross sectional area of original concrete (400 · 400 mm2 for specimen M and 250 · 250 mm2 for specimens NT, E, W and O), is the cross sectional area of the jacket concrete for specimens N, E and W, is the concrete strength of the original or the monolithic column on the day of testing and is the concrete strength of the jacket on the day of testing for the strengthened specimens NT, E and W.

The initial applied axial load for the strengthened specimens was 720.0 kN, 860.0 kN and 640.0 kN for specimens N, E and W respectively and the applied axial load for specimen M was 800.0 kN. These values correspond to initial normalized axial load ratios of 0.20, 0.21, 0.19 and 0.20 for specimens N, E, W and M respectively. Specimen O was tested by other researchers [3], who used an initial applied load of 680.0 kN. Due to the test setup, the axial load increased and decreased during testing. As shown in Fig. 6, the axial load was applied using a hydraulic jack, which was placed between the top of the specimen and

an IPE 600 steel beam. During the cyclic loading, as the specimen swayed, an increasing additional axial load was imposed via the tendons that held the IPE 600 beam in place. This resulted in an axial load that was not constant during the test. For specimens N, E and W respectively, the actual axial load ranged from 720.0 to 770.0 kN, 860.0 to 950.0 kN and 640.0 to 830.0 kN. The range of the actual axial load for specimen M was 800.0–1050 kN and the range for specimen O was from 680.0 to 690.0 kN. According to EC 2 [8], the theoretical maximum flexural strength of the strengthened specimens, if considered as monolithic, was found to be 255.5 kN m, 284.0 kN m and 260.0 kN m for specimens N, E and W respectively. These values correspond to a lateral force of 159.7 kN, 177.5 kN and 162.5 kN for specimens N, E and W respectively. The theoretical shear strengths were found to be 372.5 kN, 382.4 kN and 365.6 kN for specimens N, E and W respectively. For the monolithic specimen, the maximum theoretical flexural strength was found to be 301.0 kN m, which corresponds to a lateral force equal to 188.1 kN. In addition, the theoretical shear strength was found to be 367.8 kN. Therefore, it would be expected that the strengthened and the monolithic specimens would fail due to bending. 4. Test results 4.1. Strengthened specimens The failure mechanism of the strengthened specimens was not the same for all specimens. The typical damage sequence was as follows: horizontal cracks occurred at the beginning at the lower part of the column just above the foundation and then the cover spalled. After this point, for specimens N and E, the bond between the jacket and

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276

the original column weakened and extensive damage to the jacket was observed. For specimen W, the stirrup hook ends opened, the longitudinal bars buckled and, finally, a longitudinal bar fractured. 4.1.1. Specimen N The lateral load against displacement curve for specimen N is shown in Fig. 8. Horizontal cracks appeared quite early when the displacement was 10 mm. During the next cycle, horizontal cracks were observed 200 mm and 600 mm above the foundation and the first diagonal crack appeared on side S, 350 mm above the foundation. When the displacement was 25 mm, further horizontal cracks were observed and diagonal cracks occurred on side N. In addition, during this cycle, cracks parallel to the longitudinal bars were

200

N 150 100

Load (kN)

50 0 -50 -100 -150 -200 -150

-100

-50

0

50

100

Displacement (mm)

Fig. 8. Load against displacement curve for specimen N.

150

269

observed, which indicated a loss of bond between the bars and the jacket. At the same time, cracks appeared at the top of the jacket. During the next cycles, cracks occurred over the whole jacket length and crack widths increased. When the displacement was 60 mm, as shown in Fig. 9, a crack at the top of the jacket on side N became very wide, indicating that there was a complete loss of bond, and therefore interaction, between the original column and the jacket. This is obvious by the fact that at this point the specimen developed its maximum strength and, from the next cycle onwards, the strength quickly degraded. The test was terminated when the displacement was 80 mm because of strength degradation and extensive jacket damage. The maximum strength of the specimen was 149.8 kN. When the test was ended, the strength had reduced to 58% of the maximum. After the test, the condition of the jacket was very bad. Cracks had occurred on all four sides of the specimen and over the whole jacket height. The separation of the jacket from the original column was obvious. The jacket reinforcement bars did not buckle because the stirrup ends were welded together. After reaching the maximum strength, there was a sudden drop in strength, which was due to the separation of the jacket from the original column. The loss of bond between the jacket and the original column resulted in all the damage being restricted to the jacket, while the original column remained undamaged. The crack patterns observed on the sides of the specimen are presented in Fig. 10. The hatched areas represent areas where the concrete had spalled. 4.1.2. Specimen E The lateral load against displacement curve for specimen E is presented in Fig. 11.

Fig. 9. Crack patterns when the displacement was 60 mm.

270

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276 Push

E

Pull

Push

S

W

Pull

N

Fig. 10. Crack patterns for specimen N after testing.

200

E 150 100

Load (kN)

50 0 -50 -100 -150 -200 -150

-100

-50

0

50

100

150

Displacement (mm)

Fig. 12. Crack patterns of specimen E during testing.

Fig. 11. Load against displacement curve for specimen E.

The first horizontal cracks were observed when the displacement was 20 mm, 300 mm above the foundation. During the next cycle, additional horizontal cracks appeared on sides W and E. These cracks also developed into diagonal cracks on other two sides (N and S). At the same time, cracks parallel to the jacket reinforcement occurred and the concrete above the foundation of the specimen began to crush. When the displacement was 45 mm, a horizontal crack was observed at the base of the jacket. As the loading increased, the horizontal and diagonal cracks also increased, crossed each other and resulted in an ineffectiveness of the jacket concrete, as shown in Fig. 12. The widths of the cracks became larger and cracks were observed at the top of the jacket, as with specimen N, except the cracks had a smaller width. The bond between the original column and the jacket was not good but was better than that of specimen N, due to the presence of the dowels. Finally, the test

was terminated when the displacement was 100 mm. The maximum strength of the specimen was 162.7 kN and, when the test was ended, the strength was 66% of the maximum. The damage of the jacket was quite extensive on all four sides but the damage did not extend to the original column due to the loss of bond between the jacket and the original column. No bar buckling was observed and this can be mainly attributed to the fact that the welded stirrups did not open. Fig. 13 presents the crack patterns that were observed on the four sides of the specimen. 4.1.3. Specimen W The plot of the lateral load against displacement for specimen W is shown in Fig. 14. The first horizontal crack was observed when the displacement was 15 mm, at the base of the jacket. During the next cycles, new horizontal cracks appeared. When the displacement was 25 mm, cracks parallel to the jacket rein-

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276 Push

E

Pull

Push

S

W

271 Pull

N

Fig. 13. Crack patterns for specimen E after testing.

200

W 150 100 Bar fracture

Load (kN)

50 0 -50 -100 -150 -200 -150

-100

-50

0

50

100

150

Displacement (mm)

Fig. 15. Damaged region of specimen W.

Fig. 14. Load against displacement curve for specimen W.

forcement bars were observed. No new cracks occurred until the displacement was 50 mm but the width of existing cracks increased. At that time, the concrete started to crush and the crack at the base of the jacket was so wide that the lower jacket stirrup could be seen. After this cycle, new cracks were observed on all four sides of the specimen but their width was minimal and therefore, the concrete jacket remained effective. When the displacement was 70 mm, the first stirrup opened and when the displacement was 75 mm the first jacket bar buckled in the NW corner. During the next cycle, both bars on the E side and the bar in the SW corner also buckled. As shown in Fig. 15, when the displacement was 100 mm, the NW corner jacket bar fractured. The test was terminated when the displacement was 105 mm. The maximum strength of the specimen was 145.1 kN and the remaining strength at the end of the test was 69% of the maximum. The bond between the jacket

and the original column was very good and was significantly better than that of specimens N and E. This can be attributed to the use of shotcrete rather than cast insitu concrete. The final condition of the jacket was very good and the damage was limited to the lower part of the jacket, as also shown in Fig. 14. No obvious cracks were observed above this part of the jacket. The jacket after the testing was in much better condition than that of specimens N and E. It is worth noting that sides S and N of the jacket, which were parallel to the loading direction, had less cracks than sides W and E. The side crack patterns are presented in Fig. 16. 4.2. Control specimens 4.2.1. Specimen M Fig. 17 presents the lateral load against displacement curve for the monolithic specimen that served as one of the control specimens.

272

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276 Push

Pull

Push

Pull

Bar fracture

E

S

W

N

Fig. 16. Side crack patterns for specimen W.

200

200

O

150

150

100

100

50

50

Load (kN)

Load (kN)

M

0 -50

0 -50

-100

-100

-150

-150

-200 -150

-100

-50

0

50

100

150

-200 -150

Fig. 17. Load against displacement curve for the monolithic specimen.

The specimen failed due to bending. The test was terminated when the displacement was 100 mm because the strength of the specimen had significantly reduced. The maximum strength of the specimen was 179.0 kN and, at the end of the test, the strength of the specimen had degraded to 56% of the maximum. The external reinforcement bars did not fracture and no stirrup damage was observed. Damage was limited to the plastic hinge zone just above the base of the column. The specimen held its maximum strength for many cycles and then there was a steady strength reduction. This indicated the good ductility of the specimen. 4.2.2. Specimen O Results for an unstrengthened specimen have been taken from a previous study [3] and are presented here for com-

-100

-50

0

50

100

150

Displacement (mm)

Displacement (mm)

Fig. 18. Load against displacement curve for the unstrengthened specimen.

parison purposes. The load against displacement curve for the unstrengthened specimen (specimen O) is shown in Fig. 18. For the unstrengthened specimen, the maximum strength was 43.5 kN and, at the end of the test, the strength of the specimen had degraded to 50% of the maximum. Further details about the behaviour of the unstrengthened specimen can be found elsewhere [3]. 5. Discussion Fig. 19 presents the load against displacement envelopes for the five specimens. It can be seen from Fig. 19 that there is no significant difference between the responses in the pulling and the pushing directions. The following discussion

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276 200 150 100 F4

Load (kN)

50 F2

0 -50 -100

M N E W O

-150 -200 -150

-100

-50

0

50

100

150

Displacement (mm)

Fig. 19. Load against displacement envelopes for all specimens.

refers to pushing direction, but it must be noted that the comments made here are also valid for the pulling direction. Table 4 summarises the test results. For Table 4, the yield load, Py, and the corresponding displacement, dy, have been obtained by performing a bilinear idealization of the experimental capacity curves of the specimens. In order to establish a bilinear idealization, the rule of equal energy under the capacity curve has been adopted in a similar way to that described in ATC 40 [1], so that the total energy up to the maximum force is the same for both the experimental curve and the bilinear idealization. Values of Pmax are the recorded maximum values of the applied lateral load, while dmax are the corresponding displacements. The failure load, Pu, is defined as the lateral load that is 20% less than Pmax and the failure displacement, du, corresponds to this failure load. By comparing the results of the strengthened specimens to the unstrengthened specimen, it is obvious that concrete jacketing offers a significant enhancement to the structural characteristics (stiffness, strength and displacement capacity) of RC elements. From Table 4, the maximum strength of the strengthened specimens N, E and W were 3 to 4 times greater than the maximum strength of the unstrengthened specimen. At the maximum load stage, the displacements of the strengthened specimens E and W were more than two times greater than the corresponding displacement of the unstrengthened specimen, while the displacement of specimen N was lower than of the unstrengthened specimen. Table 4 Test results Specimen

Py (kN)

dy (mm)

Pmax (kN)

dmax (mm)

Pu (kN)

du (mm)

N E W M O

95.2 142.0 120.4 148.4 32.5

6.1 7.7 8.5 6.2 8.9

149.8 162.7 145.1 179.0 43.5

18.2 44.2 44.6 33.3 19.7

119.8 130.1 116.0 143.2 34.8

59.5 87.4 92.9 79.6 32.6

273

Specimen N was designed to prove the effectiveness of the method by reproducing an element strengthened by a jacket constructed under the worst possible construction conditions (no special preparation of the surface of the original column, no dowels or other steel connectors and a low strength poured concrete jacket). From Table 4 and Fig. 19 above, it can be seen that the maximum strength of specimen N was 3.44 times greater than that of the unstrengthened specimen. This finding demonstrates the importance of jacketing. From Table 4, when comparing strengths at the yield point stage, the strengths of specimens N, E and W were 35.8%, 4.3% and 18.9% respectively less than that for the monolithic specimen. At the maximum and ultimate load stages, these values were 16.3%, 9.1% and 18.9% respectively. As was expected, specimen N exhibited the lowest improvement in structural characteristics (when comparing the strengthened specimens). However, it must be stressed that, up to the maximum load stage, the differences were negligible. After this stage, large variations can be observed as far as the deformation capacity is concerned and this in turn led to a poor ductility performance and a premature failure. The maximum strength of specimen N occurred when the displacement was 20 mm, which was much earlier than that of the other strengthened specimens. Moreover, after reaching its maximum strength, this specimen did not maintain its strength for further cycles, which resulted in an earlier failure when compared to the other two strengthened specimens. The maximum strength of specimen N was similar to that of specimen W (which also had a low jacket concrete strength) but was significantly lower than that of specimen E. This poor behaviour as far as the ductility is concerned, may be attributed to the poor interface connection. The poor performance of specimen N cannot be considered as acceptable in earthquake loading situations. Specimen W experienced very good ductility even though the jacket concrete strength was low and there was no special preparation at the interface (as was the same for specimen N). This demonstrates the significant contribution of the steel bent down connectors. The control specimen M achieved the largest maximum load capacity and had good deformability and high ductility as, after reaching the maximum load, the specimen kept its strength for several further loading cycles. Specimens E and W performed in a similar way to the monolithic specimen by exhibiting the same deformation capacities, but they had lower strengths and stiffnesses. As specimen E was constructed without the beneficial action of sprayed concrete and without any special preparation of the surface of the original column, the good performance of specimen E, in relation to specimen W, could be attributed to the guaranteed anchorage of the stirrup ends by welding and to the presence of dowels at the interface. For comparison reasons, the results of two composite specimens [12] have been added to Fig. 19. The original

K.G. Vandoros, S.E. Dritsos / Construction and Building Materials 22 (2008) 264–276

columns of these two specimens (denoted as F2 and F4) were constructed in the same way as the original columns of the present work. The only differences were that strengthen was performed by placing two layer (F2) and four layer (F4) carbon fibre reinforced polymer, CFRP, fabrics around the original columns and the corners of columns were rounded to avoid tearing the fabric. A commercially available CFRP fabric was used and the manufactures state that the fabric has the following characteristics: a thickness of 0.13 mm, a Young’s modulus of 230 GPa, a tensile strength of 3450 MPa and an ultimate deformation of 1.5%. From Fig. 19, it can be seen that placing concrete jackets, rather than CFRP fabrics, improves the ductility and considerably improves the strength and the stiffness of the strengthened specimens when compared to the original column. On the other hand, strengthening using CFRP fabrics considerably improves the ductility since, after reaching the maximum load, it can be seen that the strength degradation was lower when compared to the concrete jacket strengthened specimens. Fig. 20 presents the stiffness degradation of the specimens. Fig. 21 demonstrates how the stiffness, K, was calculated for every loading cycle by using the following equation: Kþ þ K 2 where; K is the mean stiffness of the specimen for each cycle; K+,K is the mean stiffness in the pulling and the pushing direction respectively. These stiffnesses are calculated by dividing the maximum strength of each cycle by the corresponding maximum displacement. It can be seen from Fig. 20 that the monolithic specimen has the highest stiffness throughout the entire testing process. In addition, the stiffnesses of the strengthened specimens are quite close to the monolithic. However, specimen N experienced the highest stiffness degradation. K¼

18 M N E W O

16 14

Stiffness (kN/mm)

Failure stage

12 10 8 6 4 2 0 0

20

40

60

80

100

120

Displacement (mm)

Fig. 20. Stiffness against displacement envelopes for all specimens.

Load

274

K+

Displacement

K-

Fig. 21. Stiffness definition for every cycle.

The improvement in stiffness is obvious when comparing the strengthened specimens to the unstrengthened column, even in the case of specimen N. At all stages of testing, for the same imposed displacement, the stiffness of all the strengthened specimens is more than three times greater than the stiffness of the unstrengthened specimen. It must be noted that, at the failure stage, the stiffness of specimen N was almost 2 times greater than that of the unstrengthened specimen. When comparing the strengthened specimens, specimen E has the highest stiffness and it was the closest to the monolithic specimen. This can be attributed to (a) the higher concrete strength of specimen E and (b) minor cracking of the interface of specimen W, which occurred when the cover was chipped off in order to weld the bent down steel connectors. The initial stiffnesses of specimens E and W were found to be 14.9% and 26.6% respectively lower than that of the monolithic specimen. It is worth mentioning that both specimens experienced a lower stiffness degradation rate than the monolithic specimen. This is in agreement with the observed better ductility of these specimens, when compared to the monolithic specimen. Specimen N had the worst behaviour although its initial stiffness was only 12.7% lower than the initial stiffness of the monolithic specimen. The specimen lost its stiffness faster than the other strengthened specimens and resulted in the lowest final stiffness. This can be attributed to a lack of any interface connection between the jacket and the original column and to the use of poured concrete rather than shotcrete for constructing the jacket. As the displacement increases, the bond between the jacket and the original column is lost and slippage occurs, which results in stiffness degradation. This mechanism is the same for all the strengthened specimens. As specimen NT had the worst connection at the interface, this specimen had fastest stiffness degradation of all the strengthened specimens. The dissipated energy rates and the corresponding cumulative dissipated energy rates for all specimens are presented in Figs. 22 and 23 respectively. As can be seen

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Dissipated energy per cycle (kN*m)

20 18 16 14 12 10 8 6 M N E W O

4 2

Failure stage

0 0

5

10

15

20

25

Cycle

Fig. 22. Dissipated energy rate for all specimens.

Cumulative dissipated energy (kN*m)

180 160 140 120 100 80 60 M N E W O

40 20

Failure stage

0 0

20

40

60

80

100

120

Displacement (mm)

Fig. 23. Cumulative dissipated energy for all specimens.

275

of specimens N and E from Figs. 22 and 23, without doubt, the significantly superior performance of specimen E can be concluded. This can be attributed to the presence of the dowels at the contact surface of specimen E, since this is the main difference from specimen N. Specimen W performed better than specimen E since the use of shotcrete rather than poured concrete for the jacket resulted in higher cohesion at the interface and more friction due to the aggregate interlock mechanism between the smaller shotcrete aggregate and the surface of the original column. Moreover, the bent down bars acted as an additional energy dissipation mechanism. For these reasons, as it can be observed from Figs. 22 and 23, for nearly all stages of loading, specimen W performed better than the monolithic specimen. By looking globally at the results and by taking into account previous findings [14,15], a number of construction details can be suggested for practical applications. Firstly, in order to strengthen an element, some form of jacketing treatment is required. In the case where it is not possible to form adequate 135 angles at the ends of the stirrups, due to the obstruction of the original column, the stirrup ends must be welded together. The use of shotcrete rather than poured concrete is the preferred option but shotcrete requires the use of specialist staff and equipment. The placement of bent down steel connector bars is a worthwhile practice although the procedure is difficult to perform in practice. If bent down steel connector bars are not used for practicality reasons, the alternative of placing dowels at the interface is acceptable. Strengthening by using concrete jacket techniques considerably increases the stiffness and the strength of columns while strengthening by using CFRPs considerably increases the ductility. Finally, the experiments detailed in this paper have been carried out under laboratory controlled conditions. The engineer can only expect to achieve similar results on site by performing a very high level of quality control. 6. Conclusions

from Fig. 22, during the initial stages of loading, all strengthened specimens dissipated energy in a similar way as the monolithic specimen. However, after the 7th cycle, a faster degradation of the dissipated energy capacity is observed for specimens E and N. For these two specimens, since no roughening at the interface was performed and shotcrete was not used for the jacket, there was very little cohesion between the jacket and the original column. It appears that, after the 6th cycle, since the imposed displacement increases, the slippage at the interface increases, resulting in the loss of cohesion and a gradual separation (cycle by cycle) of the jacket from the original column. Therefore, the contribution of friction at the interface to the dissipation of energy is reduced. As expected, for the strengthened specimens, specimen N performed the worst. Nevertheless, the dissipated energy of specimen N, at the failure stage, was almost 10 times greater than that of the unstrengthened specimen. When comparing the behaviour

This paper has presented an investigation of the effectiveness of using alternative techniques to place concrete jackets in order to strengthen concrete columns. Three different jacket construction procedures were used. These were: (a) welding of the jacket stirrup ends and a poured concrete jacket (specimen N), (b) welding of the jacket stirrup ends, dowel placement at the interface and a poured concrete jacket (specimen E) and (c) bent down bars connecting the jacket bars to the longitudinal bars of the original column and a shotcrete jacket. In addition, for comparative purposes, the results from two specimens strengthened by using CFRPs have been presented. It has been demonstrated that the behaviour of elements can be significantly improved by strengthening, even when the jacket is constructed with no treatment at the interface. In effect, a lower limit to the effectiveness of the method has been set. In this case, a significant reduction in the ductility

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and the dissipated energy capacity can be expected, when compared to the other strengthening procedures. On the other hand, as far as load capacity and initial stiffness are concerned, the influence of the connection means is less significant, providing that the anchorage of the stirrup ends can be guaranteed by welding them together. When comparing specimen N to the unstrengthened specimen, specimen N achieved 3.44 times higher maximum load capacity and, at the failure stage, specimen N had 2 times higher stiffness and was 10 times better at dissipating energy. It was found that, in general, the strengths and the stiffnesses of the strengthened specimens were lower than that of the respective monolithic element. The strengths of specimens N, E and W at the yield point stage were respectively 35.8%, 4.3% and 18.9% less than that of the monolithic specimen. At the maximum and ultimate load stages, these values were respectively 16.3%, 9.1% and 18.9%. The respective initial stiffnesses were found to be 12.7%, 14.9% and 26.6% lower than the initial stiffness of the monolithic specimen. However, when special bent down steel connectors were used to connect the original column reinforcement bars to the jacket reinforcement bars, the energy dissipation rate was higher than that of the monolithic specimen. Therefore, as far as energy dissipation capacity is concerned, this technique, in combination with a shotcrete jacket, seems to be the most effective. In the case of thin jackets where the configuration of 135 hooks at the ends of the stirrups is impeded by the existing column, a significant improvement in the structural behaviour of the column can be achieved by welding of the stirrup ends together. In addition, welding the stirrup ends together stops the stirrups from opening and, in turn, the longitudinal bars of the jacket do not buckle, resulting to better maximum load capacity. Therefore, as far as maximum load capacity is concerned, the disadvantage of using a poured concrete jacket instead of a shotcrete jacket can be offset by welding the stirrup ends together. The failure mechanism and the observed crack patterns are influenced by the strengthening method. The separation of the jacket from the original column was obvious in the case where there was no treatment or other connection means performed at the contact interface between the column and the jacket. It has been demonstrated that placing concrete jackets around columns considerably increases the strength and the stiffness while placing CFRPs considerably increases the ductility.

Acknowledgement The authors wish to thank Dr V. J. Moseley for his invaluable help during the preparation of this manuscript. References [1] ATC 40. Seismic evaluation and retrofit of concrete buildings. Applied Technology Council, California, USA, 1996, vol. 1. [2] Bett BJ, Klingner RE, Jirsa JO. Lateral load response of strengthened and repaired reinforced concrete columns. ACI Struct J ACI 1988;85(5):499–508. [3] Bousias S, Spathis AL, Fardis MN. Seismic retrofitting of columns with lap-splices via RC jackets. In: Proc of 13th World Conf Earthquake Eng 2004. Vancouver, Canada: Paper No. 1937. [4] Chronopoulos M, Scarpas A, Tassios TP. Response of original and repaired reinforced concrete joints under cyclic imposed deformations. In: Proc of 10th Eur Conf Earthquake Eng, 1994. Vienna, Austria: p. 2261–7. [5] Dritsos SE, Taylor CA, Vandoros KG. Seismic strengthening of reinforced concrete structures by concrete jacketing. In: Proc of 7th Int Conf Struct Faults Repair, 1997. Edinburgh, UK: vol. 3, p. 391– 402. [6] Dritsos SE, Vandoros KG, Taylor CA. Shaking table tests on a retrofitted, small scale, reinforced concrete model. In: Proc of 6th SECED Conference on Seismic Design Practice into the next century. UK: Oxford; 1998. p. 525–33. [7] Dritsos SE. Repair and strengthening of reinforced concrete structures., Patras, Greece, 2001, [in Greek]. [8] European Committee for Standardisation (CEN). European (draft) Standard EN 1992-1-1: Eurocode 2 - Design of concrete structures Part 1-1: General rules and rules for buildings, Brussels, Belgium, 2004. [9] Ersoy U, Tankut AT, Suleiman R. Behaviour of jacketed columns. ACI Struct J ACI 1993;90(3):288–93. [10] Gomes AM, Appleton J. Repair and strengthening of reinforced concrete elements under cyclic loading. Proc of 11th European Conference on Earthquake Engineering, Paris, France, 1998, CD Proceedings. [11] Rodriguez M, Park R. Seismic load tests of reinforced concrete columns strengthened by jacketing. ACI Struct J ACI 1994;91(2):150–9. [12] Spathis AL. Experimental investigation of concrete structure seismic strengthening’’, PhD Thesis, Department of Civil Engineering, University of Patras, Greece.2006, [In Greek]. [13] Tsonos AG. Lateral load response of strengthened reinforced concrete beam-to-column joints. ACI Struct J ACI 1999;96(1): 46–56. [14] Vandoros KG. ‘‘Experimental investigation of the behaviour of columns strengthened with reinforced concrete jackets, under cyclic loads’’, PhD Thesis, Department of Civil Engineering, University of Patras, Greece, 2005. [In Greek]. [15] Vandoros KG, Dritsos SE. Interface treatment in shotcrete jacketing of reinforced concrete columns to improve seismic performance. Struct Eng Mechan J 2006;23(1):43–61.